OH Density Research Articles

Identification and imaging of OH (ν″ = 0) and O_2 (ν″ = 6 or 7) in an automobile spark-ignition engine using a tunable KrF excimer laser

Planar laser-induced predissociative fluorescence is applied to image state-specific densities of OH and hot O(2) inside an internal-combustion car engine. Improved instrumentation is described. It includes better imaging optics and a spectrometer that permits desired molecular quantum states to be selected and identified in real time. The OH (nu'' = 0) images are cleanly separated from the isooctane fuel and they display a thin superequilibrium region at the flame front. In contrast, vibrationally excited O(2) (nu'' = 6 or nu'' = 7) is uniformly distributed behind the front. Uneven and broken flame fronts are commonly observed.

The Influence of Ambient Air Entrainment on Partially Premixed Burner Flames: LIF Imaging of CO and OH

Abstract The influence of the presence and availability of ambient, “secondary” air on the oxidation of CO in partially premixed burner flames is considered, using laser-induced fluorescence. Composite two-dimensional images of the distributions of OH-radicals and CO-molecules in partially premixed bunsen flames, burning in the open air, arc compared with expectations based on the assumption of a closed system. The results indicate that many features of the structure of these flames arc intrinsically due to the interaction of the flame with the ambient environment. The imaging technique is further applied to elements from a burner system from a household appliance. In these experiments, the availability or secondary air is varied by changing the distance between two of the component burners. The images show a declining OH density between the burners as the distance decreases, with a concomitant increase in CO.

Adsorption of Citric Acid by Synthetic Pseudoboehmite

AbstractThe adsorption of citrate, at 10 4−10−3 M initial concentration, by pseudoboehmite suspended in 0.02 M NaC1O4 was investigated at varying pH. Citrate shows a strong affinity for the pseudoboehmite surface, as seen in the adsorption isotherm at pH 6. Adsorption envelopes of adsorbed citrate vs. pH for a given initial citrate concentration are characteristic for the adsorption of a polyprotic acid by a variable-charge mineral. The envelope data were fit well by the Constant Capacitance Model assuming a ligand-exchange adsorption mechanism, three monodentate surface species of citrate, and a reactive surface OH density of 0.4 mol kg−1. Aqueous speciation calculations suggest that solubility equilibrium with pseudoboehmite was attained at pH > 9 and that particulate or polymeric Al may have existed at 6 < pH < 9. Dissolved Al appeared to reduce the adsorption of citrate at pH < 5.5 via solution complexation reactions.

Local OH concentration measurement in atmospheric pressure flames by a laser-saturated fluorescence method: two-optical path laser-induced fluorescence

The first (to our knowledge) measurements of number density of OH in flames at atmospheric pressure by TOPLIF are reported. TOPLIF (acronym for two optical paths laser-induced fluorescence) improves the accuracy of LIF measurements by taking into account both the spatial profile of the exciting laser intensity and the collisional transfer rate. The method is based on simultaneously recording the LIF signals from focal volumes of two different shapes. The ratio of the signals is a measure of the saturation parameter (which depends on the laser intensity and the quenching) using which accurate determination of the species number density can be deduced from the fluorescence signals. The method is valid as far as at least partial saturation is reached. First, experimental verification of the theoretical basis of the method is reported. The population of a single rovibronic level is measured as it is in most of the spectroscopic methods. TOPLIF measures this population relative to this level's population in a chosen reference flame. Absolute value can therefore be obtained if the value in the reference flame is known or measured. Absolute [OH] profiles obtained in flat flames burning at 60 and 1000 mb are presented and compared to laser absorption measurements.

Photodissociation-region models of interstellar hydroxyl masers

Dense warm photodissociation regions are possible sites for the formation of interstellar hydroxyl masers. The column densities of OH are calculated in dense equilibrium photodissociation regions which form in the vicinity of early-type stars.

Simultaneous, in situ measurements of OH and HO2 in the stratosphere

Stratospheric OH and HO2 radical densities have been measured between 36 and 23 km using a balloon‐borne, in situ instrument launched from Palestine, TX on August 25, 1989. OH is detected using the laser‐induced fluorescence technique (LIF) employing a Cu‐vapor‐laser pumped dye laser coupled with an enclosed‐flow detection chamber. HO2 is detected nearly simultaneously by adding No to the sample flow to convert ambient HO2 to OH. Observed OH and HO2 densities ranged from 8.0 ± 2.8 × 106 and 1.4 ± 0.5 × 107 molec cm−3, respectively, at 36 km, to 1.4± 0.5 × 106 and 3.0± 1.0 × 106 at 23 km, where the uncertainty is ±1σ. The HO2 density exhibits a maximum in the 34–30 km of 1.7±0.6 × 107. The data were obtained over a solar zenith angle variation of 51° at 36 km to 61° at 23 km. O3 and H2O densities also were measured simultaneously with separate instruments.

Development of the flowing afterglow/Langmuir probe technique for studying the neutral products of dissociative recombination using spectroscopic techniques: OH production in the HCO+2+e reaction

The flowing afterglow/Langmuir probe (FALP) technique has been extended to enable the neutral products of electron–ion dissociative recombination in thermalized afterglows to be identified by spectroscopic methods. Absolute number densities of H atoms in the afterglow have been determined using vacuum ultraviolet (VUV) absorption at the Lα wavelength. By exploiting the reaction H+NO2→OH+NO, all of the H atoms can be incorporated into OH molecules and thus observation of the intensity of laser induced fluorescence (LIF) If, obtained by exciting the (1,0) band of OH(A 2Σ←X 2Π), allows a calibration to be obtained of If against the known number density of OH X 2Π(ν″=0) in the afterglow. Following this procedure, a recombining HCO+2 /electron afterglow was probed for production of ground state OH X 2Π(ν″=0) using LIF and it was established that OH(ν″=0) resulted from 17% of the recombining ground state HCO+2 ions. It was also established that a further 17% of the recombinations resulted in OH(ν″>0), i.e., that, in total, (34±6)% of the HCO+2 ions recombine to produce OH X 2Π radicals, either directly or via the electronically excited A 2Σ state. Details of the calibration procedure for H and OH number densities, of the ion chemistry involved in the production of the HCO+2 afterglow plasmas and of the checks carried out to establish that the fluorescence observed was from OH produced in the recombination reaction are presented. During these experiments, the rate coefficient at 300 K for the H+NO2 reaction was determined to be 1.3×10−10 cm3 s−1 from observations of the H-atom decay as a function of NO2 number density in the afterglow (in good agreement with previous determinations). Also the rate coefficient for the quenching reaction of OH(ν″>0) with NO to produce OH(ν″=0) was determined to be 6×10−11 cm3 s−1.

OH A2Σ–X2π(0,0) band rotational line emission rate factors

The hydroxyl radical (OH) is an important species in the study of the Earth's ozone layer, because it moderates the catalytic destruction of ozone by odd halogens and odd nitrogen. The atmospheric distribution of OH can be deduced from measurements of the ultraviolet fluorescence emission of the (0,0) band of the OH (A2Σ–X2π) system. The fluorescence emission is proportional to the product of a g factor and the OH density. Since synthetic spectral fitting of the rotational lines of the OH bands constitutes an important aspect of the measurement process used by some experimenters, accurate knowledge of the g factors for the discrete rotational lines is essential for an accurate retrieval of the OH concentration from observed spectra. We present here, for the temperatures 200.0, 240.0, 296.0, 300.0, and 500.0 K, (1) the g factor for the entire OH (A2Σ–X2π)(0,0) band and (2) the individual g factors of each rotational line for the band. The derived g factor at 240 K for the entire band is (5.5±1.1)×10−4 photon s−1.

In‐situ detection of OH in the lower stratosphere with a balloon borne high repetition rate laser system

Midday stratospheric OH density measurements have been carried out within the altitude interval of 31 to 24 km using the laser induced fluorescence technique deployed on a balloon‐borne gondola launched from Palestine, Texas on July 15, 1987. Laser output at 282 nm is produced with a pulsed, 17 kHz repetition rate, copper vapor laser pumped tunable dye laser. The OH mixing ratio ranged from 16±5 ppt at 31 km to 4±3 ppt in the 27 to 24 km region. Simultaneous ozone and water vapor measurements were also obtained with separate instruments.

Envelope structure of the Mira variable U Orionis

The authors have obtained simultaneous VLA measurements of the distributions of 1612 and 1655 MHz OH masers associated with U Orionis. At 1665 MHz they find an incomplete ring distribution near the stellar velocity of -39 km s<SUP>-1</SUP>. The observations indicate that radial expansion is the dominant kinematic component at the radius of the OH masers (≡50 AU). From consideration of all available OH, H<SUB>2</SUB>O, and SiO data, it is found that a model of axisymmetric, biconical OH density concentrations embedded in a spherical outflow at a constant velocity provides a qualitative basis for understanding the overall properties of the profile structures and the angular distributions.

Simulation of OH radical profiles in premixed atmospheric-pressure flat flames

OH profiles measured in the postflame gases of three propane-air (/phi/ = 0.63, 1.17, 1.46) flames and one (/phi/ = 1.01) methane-air flame at atmospheric pressure are compared with computer-simulated profiles generated by using a 15-reaction chemical mechanism. The simulated profiles deviate from the measured data by less than 15% for all four flames. Sensitivity analyses verify that reactions of the HO/sub 2/ radical determine the shape of the OH decay profile for the lean flame. For stoichiometric and rich flames, the radical recombination reaction (H + OH + M) strongly influences the OH radical profile. Experimental measurements of the CO, H/sub 2/, and O/sub 2/ profiles in lean and stoichiometric flames are also simulated correctly by the mechanism. For the stoichiometric flame, the experimental results verify that the H/sub 2/, O/sub 2/, and OH densities are in partial equilibrium in the postflame gases as predicted. These results provide a set of reaction rate constants and diffusion coefficients which correctly simulate the observed data over a wide range of fuel-air equivalence ratios.

Hydrogen atom abstraction by O(3P) from diborane and ethane

We measured, via LIF, time-dependent concentrations of OH resulting from the reaction of O(3 P) with B2H6 and C2H6. The oxygen atoms were generated by titrating microwave discharged N2 /He with NO to the chemiluminescent end point. The operating pressures in the flow reactor ranged from 5 to 15 Torr; the mixtures consisted of He/O(3 P)/fuel in the approximate ratios 100/1/0.1 to 100/1/1. Flow conditions were such that in the low pressure experiments the controlled residence time prior to detection were 0.8–17 ms; under the higher pressure conditions, the time interval covered was 2–35 ms. We estimated that the temperature of the reaction region was ≈350 K, based on rotational emission temperatures measured for BO*, generated under closely similar conditions. First a complete mechanism was derived for ethane for a specified set of experimental parameters. For ethane, a single set of experimental conditions was selected for ratioing the recorded intensity to the computed OH density; this was cross checked with other runs for ethane. Finally, a mechanism was developed for B2H6, which quantitatively checked our experimentally determined profiles both in shape and magnitude, for three sets of conditions, and within a factor of 2 for the high concentration runs (100% B2H6 feeding into the reactor at 14 Torr).

Mass loss from evolved stars. VII - OH maser shell radii and mass-loss rates for OH/IR stars

view Abstract Citations (107) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Mass Loss from Evolved Stars. VII. OH Maser Shell Radii and Mass-Loss Rates for OH/IR Stars Netzer, Nathan ; Knapp, G. R. Abstract The equations of photodissociation and radiative transfer are solved for the photochemical chain H2O yields OH + H yields O + H + H in the circumstellar envelopes produced by steady mass loss from cool evolved stars. The solution is performed for a large range of mass-loss rates, outflow velocities, and interstellar UV radiation field densities. Comparison with observations shows that, over a wide range of mass-loss rates, the radius at which the OH density peaks coincides with the radius defined by maser emission in the 1612 MHz line. This allows the use of these models, together with observations of OH maser shell radii and wind outflow velocities, to measure mass-loss rates. A simple power-law function relating mass-loss rate to OH shell radius and wind outflow velocity is developed from these models and is applied to the existing measurements of OH shell radii to derive mass-loss rates for 42 stars. Publication: The Astrophysical Journal Pub Date: December 1987 DOI: 10.1086/165867 Bibcode: 1987ApJ...323..734N Keywords: Interstellar Masers; Photodissociation; Stellar Envelopes; Stellar Evolution; Stellar Mass Ejection; Cool Stars; Hydroxyl Emission; Late Stars; Stellar Models; Stellar Winds; Astrophysics; MASERS; RADIATIVE TRANSFER; STARS: CIRCUMSTELLAR SHELLS; STARS: LATE-TYPE; STARS: MASS LOSS full text sources ADS | data products SIMBAD (53) Related Materials (7) Part 1: 1982ApJ...252..616K Part 2: 1983ApJ...275..330S Part 3: 1985ApJ...292..640K Part 4: 1986ApJ...303..521K Part 5: 1985ApJ...293..273K Part 6: 1985ApJ...293..281K Part 7: 1986ApJ...311..731K

Tidal influences on vertical diffusion and diurnal variability of ozone in the mesosphere

Possible dynamical influences on the diurnal behavior of ozone are investigated. A time dependent one‐dimensional photochemical model is developed for this purpose. All model calculations are made at 70°N during summer, because of available data on turbulence and zonal winds at Poker Flat, Alaska, and ozone observations from the Solar Mesosphere Explorer (SME) satellite. The model includes vertical diffusion induced by breaking gravity waves, as parameterized by Lindzen (1981). We show that the vertical diffusion can vary as much as 1 order of magnitude within a day as a result of large changes in the zonal wind induced by atmospheric thermal tides. It is found that by introducing a dissipation time scale for turbulence produced by breaking gravity waves, the agreement with Poker Flat echo data is improved. Comparisons of results from photochemical model calculations where the vertical diffusion is a function of height only with those in which Kzz is changing in time show large differences in the diurnal behavior of ozone between 70 and 90 km. By including the dynamical effect, much better agreement with the SME data is obtained. The results are, however, sensitive to the background zonally averaged wind. The influence of including time‐varying Kzz on the OH densities is also large, especially between 80 and 90 km. This suggests that dynamical effects are important in determining the diurnal behavior of the airglow emission from the Meinel bands.

Measurement of OH Density Profiles in Atmospheric-Pressure Propane-Air Flames

Abstract Hydroxyl radical (OH) density profiles as a function of height above the surface of a flat-flame burner have been determined for propane-air flames at three fuel-air equivalence ratios (0.63, 1.17, and 1.45) and for two total gas flow velocities at the richest equivalence ratio. The experiments have been performed using both absorption spectroscopy and laser-induced-fluorescence (LIF) imaging. The absorption and fluorescence profiles are in good agreement. Superequilibrium OH densities are observed near the flame zone with decay toward equilibrium occurring in the post-flame gases. An apparent second order kinetic dependence of this OH decay has been verified for the &#x1d6bd; = 1.17 flame. The rate constant for the irreversible consumption of acetylene by OH has been determined to be 1.2(+0.3)x 10-12cm3/molecule-sec in the burned gas from the richest flame for temperatures of 1620-1700 K. The data suggest that the methyl radical is an important ultimate product formed in this reaction. The observed CO d...

Determination of absolute OH and CH concentrations in a low pressure flame by laser-induced saturated fluorescence

The method of species concentration measurements by laser-induced fluorescence (LIF) in flames was further developed. The applicability of the two-level rate-equation model was examined for the experimental conditions. Inhomogeneous saturation due to the spatial intensity distribution of the laser light was corrected for using a field of local laser intensities obtained experimentally. Among other things, the accuracy of the calibration of the optical detection system was crosschecked by two methods. Absolute number densities of CH and OH could be determined in a 13 mbar C 2 H 2 −O 2 flame from measurements of the average saturation degree as a function of laser intensity. From these saturation curves approximate oscillator strengths could be obtained which provided a check of the evaluated number densities. Temperatures were determined from rotational spectra of the excited molecules. For the extension of the method to higher pressure, preliminary results obtained with a multilevel relaxation model are presented.

Spectroscopic observations of methane-pulverized coal flames

A premixed laminar flame burner was used to study physical and chemical effects of burning methane and pulverized coal simultaneously. Spectral emissions obtained as a function of height in the flame, for OH, CH, C 2, and CO were used to calculate excited state species number densities. Premixed methane-air flames with a number of equivalence ratios and methane-air-coal dust flames with three equivalence ratios were studied and differences were noted. Spatial temperatures were measured in all flames by means of a thermocouple probe. The introduction of a pulverized coal additive (30mg/min) leads to changes in OH, Ch, C 2, and CO number densities and temperature at various heights, which depend on the stoichiometry of the methane-air flame. Possible chemical and physical mechanisms responsible for these changes are discussed qualitatively. The data pose a severe test for any quantitative model which includes the fluid dynamics, gas-phase chemistry, heterogeneous processes, and radiation transport.

Time‐resolved emission studies of vibrationally excited hydroxyl radicals: OH(X2π, v″=9)

Vibrationally excited hydroxyl radicals OH(v″=9) are produced following the laser photolysis of 03/H2/He mixtures at 248 nm. The temporal profile of OH(v″=9) density was monitored by observation of the Δv=6 emission near 625 nm. Variation of the ozone pressure yields an estimate of the rate of vibrational relaxation of OH(v″=9) by 03, k ≈ 2 × 10−10 cm3 molecule−1 s−1. Comparison of the observed overtone emission intensity with that arising from the 0(21D2) precursor at 630 nm yields a factor‐of‐two estimate of the Einstein coefficient for spontaneous emission, A9→3 ≈ 0.006 s−1. This is in agreement with a previous experimental study, but does not support a theoretical value that has been extensively utilized in modeling of atmospheric Meinel emission intensity.

The photoproduction of circumstellar OH maser shells

The structure of OH shells formed from the photodestruction of H2O by ambient UV photons in the thick, expanding envelopes around cool evolved stars is investigated. The properties of the shells are governed mainly by the envelope shielding which in turn is primarily controlled by the mass-loss rate M. The peak OH densities and column densities through the shells are, respectively, slowly decreasing and increasing functions of M. The characteristic radii of the shells also depend on M, increasing from about 4(15) cm for M = 1(-6) solar mass/yr to about 1(17) cm for M = 1(-4) solar mass/yr; this dependence is well matched by recent observational data, and lends support to the OH photoproduction mechanism.

Simulation of the October 23,1980 stratoprobe flight

Measurements of ozone, water vapour, nitric acid and nitrogen dioxide taken during the STRATOPROBE flight of October 23, 1980 from Palestine Texas at 32°N are presented. The measured ratio of HNO3 to NO2 is in good agreement with that of a one dimensional model using current photochemistry. The measured ratio is used to infer "proxy" hydroxyl densities. These OH densities are very low below 25 km, in agreement with the findings of earlier flights.